Most Free Space Optical (FSO) links make use of intensity modulation or amplitude modulation of the laser light. However, the signal-to-noise ratio (SNR) of an intensity modulated optical link is limited by the shot noise in the number of photons collected at the receiver. Pulse-position modulation (PPM) can provide a means to achieve a link in which the signal-to-noise ratio can exceed the limits of shot noise. Thus, the transmitted power can be reduced, the distance of the link can be increased, the sizes of the transmitter and receiver apertures can be reduced, and the link performance can be more tolerant to the variable attenuation arising from atmospheric turbulence. Optical PPM links have been developed for transmission of digital information in long-distance or high-loss non-waveguided optical links such as deep-space links. These optical links have been demonstrated with discrete-level modulation using pulse-position modulation transmitters and photon-counting receivers that have as many as 256 levels of position modulation or 256 time slots [S. S. Muhammad, P. Brandl, E. Leitgeb, O. Koudelka and I. Jelovcan, “VHDL based FPGA Implementation of 256-ary PPM for free space optical links,” Proc. 9th Intl. Conf. Transparent Optical Networks, 2007, pp. 174-177]. Decoding the received signal typically involves detection and discernment of the presence of optical energy in the various time slots. The SNR demands for a receiver for continuous-level, analog optical PPM systems are much more stringent.
An alternative modulation scheme for an optical link uses optical analog pulse-position modulation (OAPPM) that transmit pairs of short optical pulses through free space, with the relative time-positions of, or the time-delay between, those two pulses in a pair being proportional to the time-sampled analog value of an input RF signal. As noted before, conventional optical receivers of optical links that carry intensity modulated light with direct detection of that light by a photo-detector have an output SNR that is no greater than the intensity to noise ratio of their input light. A Demodulator in an OAPPM system converts the relative time-positions of pairs of input optical pulses into an analog output voltage or current. One pulse of the pair, generally the signal pulse, has its time-position modulated and the other pulse of the pair, generally the clock reference pulse, has a fixed time position. In those cases when it is desired for the FSO link to have a lower transmit power or to cover a longer distance, the optical intensity of the received pulses can be low. Hence, it is important that the Demodulator be tolerant of intensity noise in the input pulses that are received via the free-space optical (FSO) link. Such low-intensity optical pulses may have substantial shot noise, or noise in the number of photons that comprise a pulse. An optical amplifier at the front-end of the OAPPM receiver can increase the optical power of those pulses in a pair, but that amplification process adds even more intensity noise, which could further degrade the performance of optical links that carry analog information.
A prior art OAPPM Demodulator is described in S. I. Ionov, “Method and apparatus for PPM demodulation using a semiconductor optical amplifier,” U.S. Pat. Nos. 7,605,974 and 7,330,304B2. The OAPPM Demodulator and functionality in the U.S. Pat. No. 7,330,304 is reproduced in FIGS. 1, 2A and 2B of this application. This OAPPM Demodulator accepts pairs of input optical signal and clock pulses 110 and 120, respectively, for which the variable position of one pulse in a pair (the signal pulse 110) is modulated relative to the fixed position of the other pulse in that pair (the clock pulse 120). The OAPPM Demodulator 100 needs to be able to distinguish between, and separate, the signal and clock pulses 110 and 120 of a pair. One of the prior ways to enable this separation is for the two pulses to have different optical wavelengths [see the '304 patent]. The primary component in the OAPPM Demodulator 100 is a semiconductor optical amplifier (SOA) 135 that acts as a pulse-position to pulse-intensity converter [see the '304 patent]. As shown in FIGS. 2A and 2B, one way for the SOA 135 to do this conversion is for each clock pulse 120 to deplete the carrier-population and thus the gain of the SOA 135. The carrier population then recovers gradually after that clock pulse 120 has ended because a continuous flow of carriers are supplied to the SOA 135 by means of the applied bias current. The gain experienced by the following position-modulated signal pulse 110 provides a measure of the amount of gain recovery. Thus, the intensity of the amplified signal pulse 130 is related to the time delay between that signal pulse 110 and the preceding clock pulse 120. Returning to FIG. 1, the amplified clock pulses on the output of the SOA (if any) and the amplified signal pulses 130 that are output from the SOA are separated from each other by an optical filter or demultiplexer 131 in FIG. 1. If necessary, an optical bandpass filter 137 may be used to further block the clock pulse 120. These intensity modulated signal pulses 130 are then coupled to a photodetector 140 that has a low-pass frequency response, for producing the output RF waveform while removing the aliasing spurs that are caused by the discrete-time sampling process associated with analog pulse-position modulation.
Continuing with FIG. 1, the prior OAPPM Demodulator 100 makes use of an SOA 135 as a pulse-position to pulse-intensity converter, as does the SOA of the present disclosure. However, the prior OAPPM Demodulator 100 assumes that there is a near-perfect optical limiter before the SOA 135 that removes any intensity noise in the signal and clock pulses coupled into the SOA 135. Such a near-perfect optical limiter has not been developed. Moreover, the prior OAPPM Demodulator 100 in FIG. 1 only requires that the wavelength of the signal pulse 110 and of the clock pulse 120 be different so that those two pulses can be separated. There is no requirement of any relationship between the wavelengths of the signal pulse 110 and clock pulses 120 to the gain characteristics of the SOA 135. There also is no requirement on the temporal shape of the signal pulse 110 and clock pulse 120 coupled into the SOA 135. The prior OAPPM Demodulator 100 could have a balanced pair of photodiodes comprise the photodetector 140 instead of a single photodiode. The prior art photodetector 140 with balanced photodiodes have the clock pulses 120 and the signal pulses 110 coupled, respectively, into their two optical inputs.
In the prior OAPPM Demodulator 100, the optical pulse that samples the recovering gain of the SOA 135 (the signal pulse 110) would have an intensity that depends both on the intensity of the input sampling signal pulse 110 as well as on the gain of the SOA 135 at the time of that signal pulse 110. However, because a near-perfect optical limiter is not available, the intensity of the input sampling signal pulses 110 would have substantial variations. Also, since the wavelength of the gain depleting clock pulses 120 could be longer than the wavelength of the gain sampling signal pulses 110, it actually would not be possible for those gain depleting clock pulses 120 to deplete the gain of the SOA 135 to a fixed point. Instead the depleted gain level of the SOA 135 would depend on the varying intensity of the gain-depleting clock pulse 120. As a result, the starting point for the gain recovery would fluctuate because of the noisy intensity of the gain-depleting clock pulse 120. This would transfer the intensity noise of the gain-depleting clock pulse 120 onto the modulated intensity of the gain-sampling signal pulse 110. But to achieve a desired high SNR performance, it is important the demodulation process be independent of fluctuations in the intensity of the gain-depleting clock pulses 120.